[5] The National Transportation Safety Board determined that the derailment was caused by the train's engineer (driver) becoming distracted by other radio transmissions and losing situational awareness, and said that it would have been prevented by positive train control, a computerized speed-limiting system that was operational elsewhere on the Northeast Corridor, but whose activation at the wreck site had been delayed due to regulatory requirements.

Amtrak officials said PTC had been installed on the tracks ahead of a Congress-mandated December 2015 deadline, but had yet to be operational due to "budgetary shortfalls, technical hurdles and bureaucratic rules".

For four years, the railroad struggled with the FCC to purchase the rights to airwaves in the Northeast Corridor required for PTC,[24] which might have limited the train's speed and thereby prevented the wreck.

During a press conference, NTSB member Robert Sumwalt told reporters, "Based on what we know right now, we feel that had such a system been installed in this section of track, this accident would not have occurred.

[29] Eventually, a crane was brought to the site to lift the overturned train cars, in part to search for trapped victims.

It also required Amtrak to identify other curves on the Northeast Corridor with a more than 20 mph drop in the authorized approach speed and carry out similar changes, and install additional passenger-train speed-limit signage.

[49] Bostian's lawyer said that his client does not recall much about the accident because of a concussion he sustained in the wreck, but that "he remembers coming into a curve.

[50] Earlier, Sumwalt had said, "for somebody who's been through a traumatic event, this is not at all unusual for human behavior to have the mind blank out things like that, at least for the short term.

"[1][31][51] According to The New York Times Magazine, investigators focused on two possibilities: that a rock hit the windshield of the locomotive and caused Bostian to be distracted or disoriented, or that he mistakenly believed he was in a different curve.

The board concluded that the accident would have been entirely prevented if the line had been equipped with positive train control, which would have recognized and applied the appropriate speed limit, and chairman Christopher A. Hart called on federal railroad regulators to "end this list of PTC preventable fatalities and injuries right now.

[54] President Barack Obama, in his statement following the accident, said: Along with Americans across our country, Michelle and I were shocked and deeply saddened to hear of the derailment aboard Amtrak Train 188.

Our thoughts and prayers go out to the families and friends of those we lost last night, and to the many passengers who today begin their long road to recovery.

[55]The day following the derailment, the U.S. House Appropriations Committee passed a measure to cut $260 million from Amtrak's $1.36 billion budget for the next fiscal year.

Democrat Nita Lowey said, "While we don't know the cause of this accident, we do know that starving rail of funding will not enable safer train travel."

[59] Amtrak completed installation of the ACSES positive train control system on the entire Northeast Corridor between Washington and Boston in December 2015.

[61] While § 161(a) of the Amtrak Reform and Accountability Act of 1997 (ARAA)[62] capped the total liability (including for punitive damages) of Amtrak and all other defendants in any single passenger train accident at $200 million,[63][64] a Bill (S. 1360) introduced by US Senator Bill Nelson (D-FL) on May 18, 2015, would if passed raise that limit to $500 million retroactive to May 12, 2015, the date of the Philadelphia derailment.

[65][66] If the ARAA imposed statutory liability limit proves insufficient to cover all damages and it is not raised by Congress, however, some plaintiffs' attorneys have already stated that the constitutionality of the $200 million cap would be challenged.

[67] In early July 2015, both Amtrak and the plaintiffs, in the multiple federal civil lawsuits filed against Amtrak in U.S. District Courts in Pennsylvania, New York and New Jersey, requested that all then-existing and future such actions be transferred to and heard in the Eastern District of Pennsylvania (EDPA) in Philadelphia under the management of the Judicial Panel on Multidistrict Litigation (MDL) as provided for by 28 U.S.C.

§ 1407,[68] creating a centralized process to considerably speed up litigation by conserving the resources of the parties and judiciary, avoiding duplication of discovery, and preventing inconsistent pretrial rulings.

[71][72] In answers filed by Amtrak with that Court on July 10, 2015, in response to the first two-passenger lawsuits[73] the company admitted fault by stipulating[74] as "true"[75] that the train was "traveling in excess of the allowable speed" when it derailed and thus Amtrak "will not contest liability for compensatory damages proximately caused by the derailment of Train 188 on May 12, 2015".

[76] On April 6, 2016, a Pennsylvania federal judge refused to grant settlement class certification to two passengers suing Amtrak over a train's derailment last year in Philadelphia, a ruling where damages capped at $295 million would likely be reduced in related multi-district litigation, according to plaintiffs' attorney Evan K.

[48] On February 6, 2018, following an appeal of Gehret's ruling by the Pennsylvania Attorney General's Office, Philadelphia Common Pleas Court Judge Kathryn Streeter Lewis reinstated all criminal charges against Bostian.

[84] On July 23, 2019, all charges were dismissed a second time, by Common Pleas Court Judge Barbara McDermott, who ruled that while Bostian made a mistake, his error was not a criminal act.

[87] This article incorporates public domain material from websites or documents of the National Transportation Safety Board.